Thermoelectric (TE) materials derive electrical power from a thermal gradient or create a thermal gradient from an applied electrical potential difference. They can thus serve to turn heat into electrical power or to transfer heat by applying a voltage. The first property is useful in waste heat recovery or in solar-energy conversion. In the conversion of heat energy to electrical power, in particular, TE materials offer unique advantages, including the ability to utilize the entire solar spectrum, no moving parts, and negligible cost of operation. The widespread deployment of thermoelectric systems, however, demands performance that cannot be achieved by either bulk materials or present paradigms for optimizing nanomaterials. The second property, transferring heat by applying a voltage, is the basis of TE heat pumps. These devices are useful in refrigeration and in the cooling of electronic devices. Cooling with higher efficiencies than are available presently would be valuable in cooling hot spots in integrated circuits. Advances towards higher cooling capability depend on the development of improved TE materials. Significant improvements to both heat conversion and heat removal require the development of novel functional TE materials and device structures.
The essential figure of merit for thermoelectrics, ZT, has remained near 1 in commercial materials for 40 years. Structures based on circuits using semiconductor nanostructures have exhibited values of ZT of 2 or more, and theoretical work has predicted even higher values, which have yet to be realized. Nanowires represent a tantalizing opportunity for improving thermoelectrics. Unfortunately, critical bottlenecks in applying these emerging materials have arisen in realizing circuits incorporating more than a few wires and in developing wire-based nanostructures that exhibit optimum thermoelectric properties. The growth or fabrication of structures modulated in composition is particularly challenging, but is very important if high ZT values are to be realized. The integration bottlenecks have arisen in part because conventional nanowire production has remained essentially unchanged since the vapor-liquid-solid (VLS) studies of the 1960s. The VLS process involves fundamentally stochastic phenomena, such as nucleation and growth, that are at odds with the need to create multiple identical structures, as is the norm in the semiconductor industry. The doping levels, size, and crystallographic orientation of nanowires, and even the compositions of semiconductors, must be compatible with VLS growth and cannot be optimized independently.
Superlattice nanowires are being investigated for thermoelectric applications (see, e.g., S. Lee, et al., Appl. Phys. Lett. (2006)), and it is even suspected that such structures could act as a heat engine with efficiencies near the fundamental limit set by the Carnot cycle (T. E. Humphrey and H. Linke, Phys. Rev. Lett., 94, 096601 (2005)). In a superlattice nanowire (“SLNW”) the composition varies periodically along the length of the nanowire. Current techniques for making SLNWs involve gold-catalyzed nanowire growth (M. Law, et al., Annual Review of Materials Research, 2004) or templated growth with porous materials such as alumina (L. Dresselhaus, Phys. Rev. B, 68, 075304 (2003)). The origins of the improved thermoelectric effects in these structures are still not completely understood, and it is suspected that many processes contribute to the difference between the properties of nanoscale SLNWs and larger-area superlattices. It is known, however, that the small size of the nanowires creates lateral quantum confinement and that the compositional variation creates a miniband along the length of the nanowire, providing an SLNW with controllable electronic and thermal properties. However, many SLNWs connected in series through narrow junctions are desirable for thermoelectric applications, and such connected SLNWs are impractical, if not impossible, to fabricate using conventional nanowire growth processes.